Approximating the orientation of the fault that caused the Batangas Quake - #EarthquakePH

Picture 1. Earthquake generation is usually
explainned through the Elastic Rebound Theory

April 8 this year, 3 strong earthquakes hit Batangas: a magnitude 5.7 at around 3 pm, follwed by a 5.9. These were again followed by a magnitude 5.7 within 20 minutes.

We might be familiar of the fact that earthquakes, at least the tectonic ones, happen when stored and accumulated stress on rocks finally overcome its strength, and these are associated along faults, where two rock masses move past each other. However, for the area hit, no such faults were identified at the surface, as shown by the image below.

Picture 2. Seismicity Map of Region IV - A (Source: PHIVOLCS)

Fortunately, PHIVOLCS - Seismic Observation and Earthquake Prediction Division (SOEPD) continually updates a publicly-accessible page recording data - coordinates, magnitude and depth - for every earthquake at least magnitude 1 that occurs in the Philippines. I've plotted the epicenters of all the earthquakes that have occurred around the Batangas area for the month of April, as shown below.

Picture 3. Earthquake epicenters of the area (April 2017).

As we can see above, the earthquake epicenters already show a NNW trend, which would imply that the fault that has caused these quake swarm occurs this orientation.

Now, if we plot the earthquake foci, we could see the dip, and the planar feature of this previously unknown fault. I plotted the earthquake foci using QGIS 2.8.2, with the help of the Qgisthreejs plug-in. Here is an image, showing views parallel and perpendicular to the fault's approximated strike (around N 20 W).

Picture 4. Profile showing earthquake foci for the Batangas earthquakes and aftershocks. Magnitude increases as the bubble size increases, and as color changes from blue to red. 2x vertical exaggeration. Dip angle was measured to be 88 degrees, and by quick calculation (dip angle measurements in exaggerated scales must be corrected), the dip was found to be 86 degrees.

While what I have shown lies more on an illustrative  side, allow me to take a guess, and say that the attitude of the fault that caused the Batangas quakes is around N 20 W, 86 NE.

Use of soil maps in geology

Being a student geologist from the Philippines, one of my biggest frustrations when going out in the field is the veneer of soil and 'lush' vegetation which hides the underlying lithology of the area.

This situation is unavoidable as my place, Negros Oriental, is in a tropical climactic zone - sunny, humid, and plenty when it comes to rainfall - which promotes intense chemical and biogenic weathering. This in turn accelerates the processes of soil formation.

But actually (as I have realized from one of my great epiphany moments), you could use soil maps, and a little knowledge of pedology (the study of soils), to aid you in uncovering the lithology of an area!

Here is how I intend to use this new-found knowledge in one of my assigned work in our Principles of Stratigraphy class.

As you could see below, the area I am concerned about (labelled Field Points) has a soil cover made up of the Faraon clay variety.

Picture 1. The Soil Map of South-eastern Negros

Consulting an authorized literature, such as the Simplified Keys to Soil Series - Negros Oriental (Philippine Rice research Institute, 2014), the Faraon clay series is "a calcareous, fine-textured soil with less than 65% clay, developed from the weathering of the soft and porous coralline limestones..."

Thus, without going to the field, I now have an idea about the lithology of the area I am studying, that is, it is composed of coralline limestones. This means that this area has been underwater, as coralline limestone are only deposited in shallow marine environment.

But of course, verification must be achieved if possible. Below, you could see a picture of a building stone and sand quarry in the area, which shows the subsurface strata.

Picture 2. This shows the quarry excavation found in the area. Note the thick limestone cap on top, with the thin Faraon clay soil series on top of it, as seen clearly below the bamboo. Below the limestone cap lies a thick, matrix-supported(?) calcareous conglomerate of cobble to boulder -sized clasts. Further below are interbedded alluvial sands(?) and pyroclastics. Image courtesy of Gladys Magsanay (2016).

Because of this verification, we can now infer from the soil map that those areas covered by the Faraon clay  is underlain by either a limestone cap, or a calcareous conglomerate bed.

Of course, this method of inference can only be used for residual (or authigenic) soils, which are formed from the weathering of the rock at source itself. In contrast with the allogenic soils, which are transported from another place, the soil type would tell the lithology of the source of the soil itself.

There are many other ways to incorporate soil maps in geologic analysis. Arguably, they are important tools that student geologists should be familiar about them.